Sensors can be affected by many different internal and external characteristics that can make the sensor output signals less accurate. One of these characteristics is thermal electromotive force (EMF), which relates to the effects temperature can have on the movement of electric charge in a material. A temperature gradient in a material, for example, can affect charge flow in the material much like an applied electric field, by pushing charges in a particular direction. This can be amplified in the presence of electric or magnetic fields, or concentration gradients. Thermal EMFs also can cause temperature-related charges in two primary situations: first, an inhomogeneous temperature (i.e., a temperature gradient) in a homogeneous material, or a homogeneous temperature in an inhomogeneous material. The second can occur, e.g., at device contacts, with the voltage referred to as a thermal contact voltage. Both are undesirable with respect to sensor operation and output signal accuracy.
There are many different ways in which temperature can influence charge, only some of which are related to thermal EMF. For example, magnetic sensitivity in Hall effect devices and resistivity changes due to temperature generally are not related to any thermal EMF effects and therefore may not be addressed or compensated for by embodiments discussed herein. Sensor output signals, particularly when the sensors operate according to spinning current or voltage schemes, however, can be affected by thermal EMF. In one example, a sensor system comprises Hall plates which are operated in sequential operating phases. Different terminals of the Hall plates are tapped as supply and output terminals in each operating phase, such that the current flow direction or spatial distribution of current is different from phase to phase. A spinning output signal can be obtained by combining the signals from the individual operating phases. Hall plates, in fact magnetic field sensors in general, can experience offset errors which result in an output signal when there is no applied magnetic field. Offset errors in each operating phase can be largely canceled in spinning schemes due to the combination of the individual operating phase signals, such that little or no residual offset remains in the combined output signal.
Unfortunately, residual offset errors often remain, such that some spinning scheme sensor systems provide residual offset compensation. Referring to FIG. 1, such systems typically comprise a temperature sensor arranged in close proximity to the Hall plate because offset correction usually is not constant over temperature. The system therefore can sense the temperature, determine a compensation signal based on the temperature, and take this compensation signal into account in the spinning output signal. Thus, this conventional approach combines the phase temperature signals only by averaging, which can be considered equivalent to an implicit low-pass filtering of a slow temperature sensor. A challenge, however, is determining the compensation signal. Because the residual offset of spinning Hall schemes is stochastic, it depends on the actual individual device, and the temperature of this device, and it can change during the operational lifetime of the device. Thus, even if individual device calibration could be performed efficiently and effectively during end-of-line testing, changes over the lifetime of the device can reduce the accuracy of the calibration and result in thermal EMF-related residual offset errors.
Conventional solutions presume that thermal EMF effects are canceled through use of polarity inversion in sequential operating phases (i.e., only the polarity of the supply changes) and a spinning voltage rather than current technique. This may not be the case, however, because in practice the temperature distribution can change when the polarity of the supply voltage is inverted.